Taking Adult Learning Principles Into Account
While adult learning principles tend to be over-rated by trainers, and do not necessarily reflect real learners trainers face, it's worth considering them. Consider, for example, that adults can use their past experience to help them learn new things, and anchor new learning. So, one factor in choosing an instructional method may be the degree to which it allows learners to draw upon their experiences and bring them to bear on what is to be learned.
Ensuring Interaction Occurs
Interaction is a critical component of learning. For example, a trainer who interacts (talks with) learners as part of the instructional strategy is going to be better at identifying how learning is going, and whether things need to be altered. Interaction is also an active approach which tends to encourage learners to think. While not all learning activities require interaction, certainly it's important in anything but the shortest sessions.
Ensuring An Effective Workshop Pacing
Pacing is a term used to describe changes in "speed" during instruction, which ties into attentional issues. For example, lecture is a "slow" method, while brainstorming should be a "fast" method. Slow methods are best for allowing and encouraging in depth thinking, while fast methods are good for idea generation, etc. Learner attention is best achieved through the use of different speeds, so again instructional method variety is important here.
Other factors can also be considered -- costs, availability of technical resources, etc.
As you can see, the choosing of the right instructional methods is really a balancing act, where different factors need to be considered and waited. The training designer, or trainer, has to exercise judgment, both in the design process, and in delivering. Often, trainers profit from having various activities to achieve the same objectives, so the delivery can be modified on the fly.